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Headphone Impedance

Headphone Impedance is the electrical characteristic of the headphone voice coil and magnetic field coupling of the voice coil & magnet inside the headphone.

Impedance is an electrical unit which expresses the combined Resistance, Inductance and Capacitance of the headphone's voice coil.

In terms of matching to an amplifier, the headphone impedance is the "Load Impedance".

An amplifier drives a headphone with voltage, the current drawn by the headphone is proportional to the headphone's impedance.

As a rule of thumb, the load impedance (headphone) should be at least eight times higher than the amplifier output impedance.

This allows the amplifier to exert more control over the headphone, i.e. the lower the output impedance of the amp, the higher the damping factor and the higher the efficiency of the amplifier/headphone interface (See note 1 for additional detail).

Headphone impedance is usually increased by thinner wire and most importantly more turns of wire in the voice coil. More turns or loops creates a larger field (area of magnetic influence). In layman's terms more magnetic force for the coil to move the diaphragm. Thinner wire usually works out to a lighter, more responsive diaphragm. Depending on the headphone design, this may lead to more accurate response.

The displacement (amount of movement) of the diaphragm (the part that vibrates to produce sound) can be better controlled via a more accurate flux (magnetic field to pull and push the diaphragm).

Difficult to drive for small headphone amps with low output voltage and low gain.

Allow Solid State Op Amps to work more efficiently with less distortion. Have a look at Op Amp data sheets and a graph of distortion vs. output impedance for most audio Op-Amps and you'll get the idea. This is a very complex subject, but most Op Amps are designed to output (typically) up to 10 Volts into 600 ohm loads or higher.

Pros and Cons of low impedance headphones:

Headphone impedance is usually decreased by thicker wire and less turns of wire in the voice coil. The magnetic field is built up by more current.

Easier for small and/or portable headphone amps to drive. For example: an iPod or MP3 player headphone jack. Many small and/or portable headphone amplifiers are designed to output a volt or two into low impedance, high efficiency headphones, e.g. Grado headphones.

Low impedance headphones usually sound better when driven by a solid state or a transformer coupled vacuum tube amplifier. Low impedance headphone do not usually work well with Output Transformerless vacuum tube amps.

Generally, a properly designed desktop headphone amplifer can drive high and low impedance headphones and can drive headphones of any efficiency.

Do high impedance headphones sound better than low impedance ones?

No, there are many very good low impedance headphones available and many very good high impedance headphones available.

In headphones, the sound also depends upon the following:

Frequency response, this is a representation of volume decreasing or increasing with frequency, this is actually the magnetic field pulling and pushing with different amount of force at different frequencies, impedance affects this in complex ways.

Distortion, represents the amount of "change" from the actual signal to the real signal (i.e. input signal to output signal).

Build and design, i.e. sealed vs. open, etc.

Diaphragm design, voice coil design

To conclude:

High impedance headphone: requires more voltage but less current

Low impedance headphone: requires less voltage but more current

Amplfier output impedance: the lower the better for any impedance headphone

higher efficiency (for example: 102 dB/1 mW) less power required

​lower efficiency: (for example: 91 dB/mW) more power required

How they effect sound is dependent on the magnetic field generated and diaphragm construction (physical response of diaphragm).

We hope this clears the air.

A few notes:

Some people prefer the sound of vacuum tube headphone amplifiers which usually have higher output impedance relative to solid state (transistor) amplifiers, and therefore have poorer damping factor and hence, fuller bass.

So does this mean that even lower impedance headphones would benefit from a headphone amplifier because it could deliver more, though slower, electrons? My Denons AHD2000s are only 25 ohms, but I couldn't imagine listening to them without a headphone amp.

Do you know any good sources (papers, websites, books) of more information on headphone design, impedance, and the effects of amplification? I've had several undergraduate courses in EE and signals but never a really in-depth explanation of the physics of dynamic speakers. If my schedule wasn't so packed with computer engineering courses I would take some audio engineering courses, but alas time is not on my side right now.

The speed of electrons never changes, as a function of impedance or anything else. It's a constant. Also, the planar magnetics that are so popular in recent times show that low-impedance cans certainly can deliver subtlety. From time to time, ribbon speakers become all the rage in high-end hi-fi, usually for their "unbeatable" clarity, delicacy, etc. The same is said of electrostats, and so on. We need to be cautious of generalizations, especially when answering noobie questions..

PS: In the post above, I meant to point out that speakers like the Apogee Scintilla and Martin-Logan CLS approached a "dead short" (short-circuit) at high frequencies, yet each of these was all the rage for a period and considered state-of-the-art by its proponents. Farther back then that, electrostatic speakers with no input transformers were driven by direct-coupled OTL tube amps, which mated well because of the stat's multi-thousand-Ohm impedance. So, again, high or low impedance, transformer coupled or direct-coupled, solid state or tubes... all can work exceedingly well if properly designed and well engineered. Due diligence in research can protect against disappointments down the road.

i agree need to edit this, speed of electrons is constant in a conductor, its the kinetic energy which is responsible. The amount of work done in moving an e- form 1 point to another is voltage not it's relative speed. jeffreyfranz, it's a wiki please edit as you like.

I am glad that this discussion has been helpful. However, I do not have a great deal of knowledge beyond what I have already shared. I am not an EE. What would be good for a true Wiki reference on this subject (and related ones) is for a veteran engineer with a talent for expressing complex subject matter in common language to lend a hand.

This is a simpler and practical expression and takes in a lot of assumptions, one could go onto more depth but then it wont be useful for the people who don't have an EE background. I kept it short for this reason only.

Isn't it so that a headphone amplifier has an output impedance, and that it should be matched as close to the headphone impedance as possible to ensure the best damping factor - but then what is "output impedance"

Ardilla, I have edited this to try an answer your question.There is more detail in the article, but generally speaking, output impedance of amp should be much lower than impedance of headphone you are using.

The whole idea is to run things in their comfort zone, for an amp output impedance 600 Ohm driving a 600 Ohm headphone is a full workout for the amp, its a nice idea to have some headroom for the amp to run smoothly.

To follow up on Ardilla's question, more specifically: would a pair of Sennheiser HD 650s at 300 ohms be a good match for an amp like the Musical Paradise MP-301 MK2 with an output impedance of 4-8 ohms?

JackonStone:Yes.An amplifier with an Output Impedance of 4-8 ohms is fantastic for 650 ohm headphones. When it comes to output impedance, the lower the better. This is because headphone amplifiers are voltage sources and you are trying to get as much voltage from the amp to the 'phone as you can possible get.Where does power fit into this? Power is a function of voltage and impedance.For a 650 ohm headphone you want an amp which can output a fair amount of voltage, a desktop amp will usually output more voltage than a portable amp. You should be OK with that amp.BTW, I love tubes, but that is a subjective statement!